I like to tear it all apart and clean it up before welding.  It's nice to have the ring gear out of the way.  It helps keep slag out of the recesses of the diff.  You can do a better job of it if you do.  Setting backlach is not big deal and can be done with a cheap dial indicator.  Probably not a big deal but that's how I like to do it and I haven't had any break that way.